JSFiddle - React, Tailwind, and code Playground

HTML

<div id="ContentMain">
    <div id="Banner">Banner goes here</div>
    Content goes here
    
    </div>

CSS

body
          {
            background-color: #E7E7E7;
            text-align: center;
            font-family: Arial, Helvetica;
            font-size: 15px;
            color: #000000;
            border-spacing: 0;
            border-collapse:collapse;
            padding: 0;
            margin: 0;
          }
          
          #Banner {
            background-color: #333FFF;
            top: 0; /* Probably not necessary... */
            height: 40px;
            width: 100%;
          }
    
          #ContentMain
          { 
            background-color: #FFFFFF;
            height: 100%;
          }